The present invention relates to a method of cleaning chambers that are used in the processing of substrates.
In the manufacture of integrated circuits, materials such as silicon dioxide, silicon nitride, polysilicon, metal, metal silicide, and monocrystalline silicon, that are deposited or otherwise formed on a substrate, are etched in predefined patterns to form gates, vias, contact holes, trenches, and/or interconnect lines. In the etching process, a patterned mask composed of silicon oxide or silicon nitride (hard mask) or photoresist polymer, is formed on the substrate by conventional photolithographic methods. The exposed portions of the underlying material that lie between the features of the patterned mask are etched by capacitive or inductively coupled plasmas of etchant gas. During the etching processes, etchant residue (often referred to as a polymer) deposits on the walls and other component surfaces inside the etching chamber. The composition of the etchant residue (residue from the etch process) depends upon the chemical composition of vaporized species of etchant gas, the material being etched, and the mask layer on the substrate. For example, when tungsten silicide, polysilicon or other silicon-containing layers are etched, silicon-containing gaseous species are vaporized or sputtered from the substrate; similarly, etching of metal layers results in vaporization of metal species. In addition, the mask layer on the substrate is also partially vaporized by the etchant gas to form gaseous hydrocarbon or oxygen species. The vaporized and gaseous species condense to form etchant residue comprising polymeric byproducts composed of hydrocarbon species from the resist; gaseous elements such as fluorine, chlorine, oxygen, or nitrogen; and elemental silicon or metal species depending on the composition of the substrate being etched. The polymeric byproducts deposit as thin layers of etchant residue on the walls and components in the chamber. The composition of the etchant residue typically varies considerably across the chamber surface depending upon the composition of the localized gaseous environment, the location of gas inlet and exhaust ports, and the geometry of the chamber.
The compositional variant, non-homogeneous, etchant residue formed on the etching chamber surfaces has to be periodically cleaned to prevent contamination of the substrate. Typically, after processing of about 25 wafers, an in-situ plasma “dry-clean” process is performed in an empty etching chamber to clean the chamber. However, the energetic plasma species rapidly erode the chamber walls and chamber components, and it is expensive to replace these parts and components. Also, erosion of the chamber surfaces can result in instability of the etching process from one wafer to another. The relatively thin and compositionally variant etchant residue can also make it difficult to stop the in-situ plasma clean process immediately after all of the etchant residue is removed—which results in erosion of the underlying chamber surfaces. Also, it is difficult to clean-off the chemically hard residue deposited at portions of the chamber surfaces without entirely removing chemically softer residues at other portions of the chamber and eroding the underlying chamber surfaces. For example, the etchant residue formed near the chamber inlet or exhaust often has a higher concentration of etchant gas species than the residue formed near the substrate which typically contains a higher concentration of resist, hard mask, or of the material being etched.
It is difficult to form a cleaning plasma that uniformly etches away the compositionally different variants of etchant residue. Thus after cleaning of about 100 or 300 wafers, the etching chamber is opened to the atmosphere and cleaned in a “wet-cleaning” process, in which an operator uses an acid or solvent to scrub off and dissolve accumulated etchant residue from the chamber walls. To provide consistent chamber properties, after the wet cleaning step, the chamber and its internal surfaces are “seasoned” by pumping down the chamber for an extended period of time, and thereafter, performing a series of runs of the etch process on dummy wafers. The internal chamber surfaces should exhibit consistent chemical surfaces, i.e., surfaces having little or no variations in the concentration, type, or functionality of surface chemical groups; otherwise, the etching processes performed in the chamber produce varying etching results from one substrate to another. In the pump-down process, the chamber is pumped down to a high vacuum environment for 2 to 3 hours to outgas moisture and other volatile species trapped in the chamber during the wet clean process. Thereafter, the etch process to be performed in the chamber, is run for 10 to 15 minutes on a set of dummy wafers, or until the chamber provides consistent and reproducible etching properties.
In the competitive semiconductor industry, the increased cost per substrate that results from the downtime of the etching chamber during the dry or wet cleaning and seasoning process steps, is undesirable. It typically takes 5 to 10 minutes for each dry cleaning process step, and 2 to 3 hours to complete the wet cleaning processes. Also, the wet cleaning and seasoning process often provide inconsistent and variable etch properties. In particular, because the wet cleaning process is manually performed by an operator, it often varies from one session to another, resulting in variations in chamber surface properties and a low reproducibility of etching processes. Thus it is desirable to have an etching process that can remove or eliminate deposition of etchant residue on the chamber surfaces.
In semiconductor fabrication, yet another type of problem arises in the etching of multiple layers of materials that have similar constituent elements, for example, silicon-containing materials such as tungsten silicide, polysilicon, silicon nitride, and silicon dioxide. With reference to FIGS. 1a and 1b, a typical multilayer polycide structure on a semiconductor substrate 25 comprises metal silicide layers 22 deposited over doped or undoped polysilicon layers 24. The polycide layers are formed over silicon dioxide layers 26 and etched to form the etched features 29. In these multilayer structures, it is difficult to obtain a high etching selectivity ratio for etching the metal silicide layer 22 relative to the overlying resist layer 28 or the underlying polysilicon layer 24. It is desirable to have a high etching selectivity ratio for etching polycide structures that have a non-planar and highly convoluted topography. At a certain time during the etching process, the thinner metal silicide layer 22 is etched through and etching of the underlying polysilicon layer 24 begins, while the thicker metal silicide layer 22 is still being etched. Thus, it is desirable to etch the metal silicide layer 22 at a faster rate relative to the rate of etching of the polysilicon layer 24, for example. The same problem arises in the etching of a mask layer of silicon nitride 32 on a very thin silicon dioxide layer 34, prior to forming trenches in a substrate comprising silicon 36, as for example shown in FIGS. 1c and 1d. The etched trenches 38 are used to isolate active MOSFET devices formed on the substrate. The etching selectivity ratio for etching silicon nitride relative to silicon dioxide has to be very high to stop on the silicon dioxide layer 34 without etching through the layer.
High etching selectivity ratios are obtained using a process gas composition that etches the different silicon-containing materials at significantly different etching rates which depend upon the chemical reactivity of the particular process gas composition with a particular layer. However, etching metal silicide layers with high selectivity to polysilicon, or etching silicon nitride layers with high selectivity to silicon dioxide layers, is particularly difficult because both materials contain elemental silicon and most conventional etchant plasmas etch the silicon containing layers to form gaseous SiClx or SiFx species. Thus, it is difficult for the etchant plasma to chemically distinguish and preferentially etch the metal silicide layer 22 faster than the polysilicon layer 24, and the silicon nitride layer 32 faster than the silicon dioxide layer 34. This problem is further exacerbated because the etchant residue formed on the chamber sidewalls also contains silicon dioxide and attempts to remove the etchant residue during the polycide etching process can lower the etching rate or etching selectivity ratio for etching these layers.
Thus it is desirable to have an etch process that reduces formation of etchant residue in the etching chamber. It is also desirable if the etchant or cleaning gases do not to erode the exposed surfaces in the chamber. It is further desirable to have an etching or cleaning process that restores the original chemical reactivity and surface functional groups of the chamber surfaces. It is further desirable for the cleaning process to remove chemically adhered etchant residue layers having variable thickness and variant chemical compositions and reactivity without excessive erosion of underlying chamber surfaces.